Particularly in two state or on/off control systems but also in proportional and more sophisticated control systems, overshoot is a common but unwanted operational reality. Overshoot may generally be defined as an undesirable and excess response to a control signal resulting in the controlled variable temporarily exceeding or overshooting the new, desired or target controlled value. The analysis of control overshoot and undershoot will not be addressed here beyond the acknowledgement that while overshoot or undershoot are generally undesirable and are to be minimized, such minimization carries with it compromises such as reduced speed of response and steady state errors, to name but two.
Control errors such as overshoot reside in many control systems, especially those associated with massive, mechanical devices. The manufacturer of motor vehicles and particularly heavy duty automated truck transmissions are often faced with control and overshoot challenges. Clearly, rapid, smooth and positive gear shifts are a most desired goal. However, each engine (and its electronic controller) with which a truck transmission may be mated will have slightly different speed, power and torque versus time characteristics. For example, in response to a throttle position change, one engine may accelerate and decelerate differently from another engine and may exhibit these differences in a distinct manner across various regions of the speed, power and torque curves.
For example, a command to one type or brand of engine to increase its speed from 1500 to 2000 rpm may achieve a first grouping of values of acceleration, elapsed time, overshoot and time to final, steady state speed, while another equally suitable type or brand of engine will exhibit another quite distinct grouping of values.
One of the significant areas of performance difference which implicates both the engine and its electronic control is overshoot, i.e., the tendency, upon receipt of a speed change command, to briefly exceed or overshoot either in a positive or negative direction, the new or target speed value. Such overshoot, if unaddressed, may result in an apparently poorly executed shift. For example, if a transmission/clutch controller determines during a downshift that the master clutch will be engaged when the engine speed 2000 rpm, the transmission/clutch controller will track the increasing engine speed and determine that at a specific future time, the engine speed will be 2000 rpm. Since at that specific future time, the engine speed will match the transmission input shaft speed in the newly selected gear, the master clutch should be engaged. Unfortunately, due to overshoot, the engine speed may briefly rise to 2050 rpm or 2075 rpm and then decay to 2000. If clutch engagement occurs above the 2000 rpm target speed and especially if it engages at or near peak rpm of 2075 rpm, a perceptible lurch will be experienced by the vehicle operator. Beyond momentary operator and passenger discomfort, such a lurch is indicative of a driveline torque surge and results in stress on the driveline components, especially the master clutch, which is highly undesirable. The present invention addresses the problem of engine/controller overshoot and detects the actual overshoot of an engine/controller combination and compensates for such overshoot.